1. Field of the Invention
The invention relates to an optical disc apparatus which can record, reproduce, or erase information signals onto/from both of an optical disc having a recording density similar to that of a conventional CD (compact disc) and an optical disc having a recording density higher than the above recording density.
2. Description of the Prior Art
In recent years, in addition to an optical disc apparatus only for reproduction such as a CD player or the like, an optical disc apparatus which can record and reproduce an information signal is actively being developed.
Ordinarily, the recording and reproduction of an information signal onto/from an optical disc are executed by converging a beam which is radiated from a semiconductor laser or the like onto a recording layer of the optical disc by a lens. The recording layer here denotes a pit layer in the case of a CD and is a layer in which a deformation, a change in optical constant, a formation of a magnetic domain, or the like is performed by a converged laser beam in the case of a recordable optical disc. To raise a recording density of the optical disc, it is necessary to reduce a spot diameter D of the converged beam. There is the following relation among the spot diameter D, a numerical aperture NA of the lens, and a wavelength λ of the laser beam.                     D        ∝                  λ          NA                                    (        1        )            
The above equation (1) denotes that the beam spot diameter D decreases by using a lens of a large NA. That is, by increasing NA, the high density recording can be executed.
When NA of the lens increases, however, an aberration of the converged beam due to an inclination error of the disc called a tilt increases. Particularly, a coma aberration increases. There is the following relation among a wave front aberration Wc of the coma, a tilt angle α, and NA when using a thickness d and a refractive index n of the disc substrate.                               W          c                ∝                                                            n                2                            -              1                                      2              ⁢                              n                3                                              ·          d          ·          α          ·                                    (              NA              )                        3                                              (        2        )            
The above equation (2) denotes that in the case of using a lens of NA which is larger than that of the conventional lens, even if a tilt angle is identical, the coma aberration increases. It will be understood from the equation (2), however, that there is an effect to suppress the coma aberration by setting the thickness d of the disc substrate to be thin. In the optical disc for the high density recording, therefore, it is preferable that the thickness of the disk substrate is thinner than that of the conventional optical disc, so that an optical head using an objective lens corresponding to the thin disc substrate is needed.
On the other hand, even in the optical disc apparatus corresponding to the high density recording, it is preferable that the conventional optical disc of a thick substrate can be also reproduced so that a great amount of conventional software resources can be utilized.
However, the optical head which has been designed for a thin substrate cannot be used for an optical disc of a thick substrate. The reasons will now be described hereinbelow. The objective lens for an optical disc has been designed so as to set off a spherical aberration which occurs when the converged beam passes in the disc substrate. Since such an aberration is corrected in accordance with the thickness of the disc substrate, the aberration correction is not accurately performed for the converged beam which passes through the disc substrate having a thickness different from the design value. The above point will now be explained with reference to the drawing. FIGS. 18A and 18B are schematic side elevational views for explaining a situation of the occurrence of the aberration due to the disc substrates having different thicknesses. FIG. 18A is a diagram in the case of using an objective lens which has been designed for a thin disc substrate and shows traces of lights in a state in which a beam has been converged through the disc substrate having the same thickness as the design value. In the diagram, a broken line indicates the surface of a recording layer and all of the lights emitted from the objective lens are converged to one point 0 on the recording layer surface. FIG. 18B is a diagram in the case of using an objective lens which has been designed for the same thin disc substrate as that of FIG. 18A and shows traces of lights in a state in which the beam has been converged through the disc substrate having a thickness which is thicker than the design value. In FIG. 18B, the lights emitted from the outermost peripheral portion of the objective lens are converged to a point O′ on the recording layer surface. However, the light locating near the optical axis is converged at the front side. Such a phenomenon is called a spherical aberration. Whenever the term “aberration” is used hereinafter, it means spherical aberration. When such an aberration occurs, the objective lens cannot converge the light beam until what is called a diffraction limit. Therefore, in the case of using the objective lens whose aberration has been corrected for a thin disc substrate, an information signal cannot be recorded, reproduced, or erased onto/from an optical disc having a thick disc substrate. Similarly, in the case of using the objective lens whose aberration has been corrected for a thick disc substrate, an information signal cannot be recorded, reproduced, or erased onto/from an optical disc having a thin disc substrate.